An organic photoconductor typically comprises an anodized layer on a conductive subtrate such as aluminum drum or a barrier layer, a charge generation layer (CGL) and a charge transport layer (CTL). The charge generation layer is made of a pigment, such as metal free or metal-phthalocyanine, squaraine, bisazo compound or a combination of a bisazo and trisazo compounds. The mechanical integrity to a charge generation (CG) layer is often derived from a polymeric support. Various polymer binders have been used for this purpose. Some of these polymers are polyvinylbutyral, polycarbonates, epoxy resin, polyacrylate, polyesters, phenoxy resin, phenolic resins to name a few. In the case of phthalocyanines, polyvinylbutyrals (PVBs) have been the polymers of choice. This polymer may be used in combination with other polymers. Although the patent literature abounds in a number of publications with respect to the use of PVB, and a mention of phenoxy resin as a supporting binder, no mention is made of the role of the phenoxy resin, or the use of blends corresponding to phenoxy, epoxy or epoxy novolac resin with PVB. This invention focuses on the improved electrical properties derived from the phenoxy resin, epoxy resin or epoxy novolac resins, when used as a supporting polymer binder in the charge generation layer. The improved electrical characteristics relate to improved dark decay and sensitivity, while retaining good adhesion (with respect to PVB) and structural integrity.
Phenoxy resins have been reported to serve as polymer binders for bisazo pigments. EP 708 374 A1 (1996) demonstrates the use of a phenoxy resin as a binder for a bisazo pigment. The authors refer to some agglomeration in some of the dispersion formulation based on the phenoxy resin, but attributed this to the combination of the coupler residue and the azo pigment. Some of the other patents relating to either a use or a possible use of the phenoxy resins as binders with bisazo pigments are JP 03158862 A (1991), JP 03116152 A (1991) and JP 01198762 A (1989). The Japanese patent 03282554 A (1991) demonstrates the use of a phenoxy resin as a binder for a metal-free phthalocyanine and using 1,1,2-trichloroethane as a solvent. Other patents pertaining to the phthalocyanine based phenoxy resin formulations include JP 02280169 A (1990), GB 2 231 166 A (1990), U.S. Pat. No. 4,983,483 (1993) to name a few. Limburg et al. (EP 295 126 A2, 1993 and U.S. Pat. No. 4,818,650, 1987 have discussed the use of a polyarylamine phenoxy resin as part of the charge transport layer in the preparation of a photoreceptor. The above photoreceptor was shown to exhibit improved resistance to cracking during mechanical cycling. Phenoxy resin based polymers have also been used as undercoats in the preparation of photoconductors (e.g. JP 03136064 A, 1991).
The use of the phenoxy resin as a binder is hence fairly well known. However, it was surprising to note in this invention, that the phenoxy resin can be used to improve the electrical characteristics of the photoconductor. The use of the phenoxy resin as blends results in improved electrical characteristics, without having to increase either the pigment or charge transport molecule concentration, which in turn relates to lower cost of the resulting photoconductor drum. The use of the polymer in formulations investigated in this invention has not been reported in the patent literature. The importance of this invention can be extended to the use of the phenoxy resins in the preparation of photoconductors required for high speed printer applications which would require high sensitivities, low dark decays and use in any environmental condition (ambient, hot/humid or cold/dry). The dark decay for these formulations improve by 5-40%, the change in electricals in various environments is usually less than 35 V and the sensitivity measured at most energies are improved, in comparisons to photoconductors comprising of PVB as CG binder only.
The inventors found that the use of the phenoxy resins as a pure binder results in highly unstable dispersions of the titanyl phthalocyanine and hence cannot be used in the coating process of photoconductor drums. However, the use of the phenoxy resins as a blend with PVB, results in stable dispersions and the resulting photoconductor drums are found to exhibit superior electrophotographic properties such as low dark decay's and high electrical sensitivity's.
The use of epoxy novolac resins as a binder polymer in the charge generation layer of an electrophotographic photoreceptor is not known in patent literature. Epoxy resins have been used in the preparation of barrier layers, adhesive layers and charge generation layers. In a similar manner, phenolic resins have been shown to improve the adhesion of the CG layer to the aluminum core. Epoxy-novolac resins are essentially a combination of the epoxy resins and the phenolic resins. The resin system can be cross-linked either chemically or thermally. The cross-linked resins usually result in enhanced mechanical properties in comparison to their precursors. The thermal cross-linking reaction can essentially be brought about during the curing of the CG layer. The chemical cross-linking may be brought about by the addition of catalysts such as titanium alkoxides. The epoxy functionality and the phenolic functionality not only impart good mechanical integrity to the charge generation binder, but also improved adhesion of the CG layer to the aluminum core.
Several patents in the literature refer to the epoxy resins as possible in binders in the sub-layer, charge generation or charge transport layers. For example, U.S. Pat. No. 5,240,801, 1993 lists the epoxy resin as a polymer for a protective coat layer. JP 621194257 A, 1987 suggests the use of epoxy resin as a binder for an oxazole charge transport molecule. EP 180 930 A2 (1986) (Mitsubishi Chem. Ind.) lists several binders for CG layer of which polyvinylbutyral and epoxy resin are two of them. JP 56097352 A, 1981 lists binders in a general manner as those derived from addition or condensation reactions and refer to the epoxy resins as an example.